Chillers under control

JULIAN MILLER
Chillers operating 24 hours a day, seven days a week, is normally a bad sign — Julian Miller.
JULIAN MILLER believes that many chillers are allowed to operate very inefficiently and suggests approaches for reducing their energy consumption. There is little logic behind many control strategies for chilling. By applying some, significant savings could result.The middle of Winter might seem a strange time to be considering chillers, but how many chillers out there remain running despite a net heating demand? For that matter how many boilers continued running throughout the Summer regardless of heating demand? Conflict Logically, the two are in direct conflict with each other, so running them at the same time must be illogical and you don’t need the Vulcan brain of Dr Spock from Starship Enterprise to work that one out! Logic and common sense play a significant part in improving the efficiency of building-services plant, so why is it such a problem? One of the greatest causes of energy inefficiency in buildings is the conflict between heating and chilling systems — and it really should not be so. The good news is that such conflict is not a difficult problem to overcome, given a little focus, and the result will lead to significant energy savings. As more and more buildings have chillers installed the problem is growing and working against our efforts to reduce the emissions of carbon dioxide and other greenhouse gases. To begin with, if you are installing chillers, ensure they are not oversized. Oversized chillers not only cost more to install, they will also cost much more to run over their life time. In fact when you install chillers (or if you already have them) make sure that you meter their electrical consumption so you know how much they are using. Chillers probably account for up to a third of your total electrical cost, but if you do not know how much energy they use you will never be able to manage them effectively. Simple opportunities There are a few simple opportunities for ensuring that chillers will not run when they are not required; First, get control of your chillers (and boilers) onto your building and energy-management system, or you will never know if supply meets demand. Many chillers come with their own control sets, and these are perfectly adequately for providing the right temperature. They will not, however, optimise efficiency as they do not know what any of the rest of the building’s plant is up to. Chillers operating 24 hours a day, seven days a week, is normally a bad sign at any time of year. Even in the very hot conditions of August 2003 it was not necessary to chill most buildings for a full 24 hours. When temperatures fell overnight it should have been possible to use a night purge, bringing in outside air to cool the building in readiness for the following day. The night purge should be standard in all situations. Why pay for artificial chilling when it is available free from the great outdoors? Make sure that your chillers are not inadvertently acting as heaters. This may sound strange, even illogical, but on more than a few occasions we have seen chillers producing flow temperatures above the outside-air temperature, in effect the chillers are taking air in air at, say, 6°C and ‘warming’ it to a flow-temperature set point of 10°C to distribute throughout the building. Why not just bring in the fresh air and distribute it through the air handlers, heating it with the boilers if required. This makes more sense, and I think our friend Dr Spock would be more comfortable with this arrangement! Sequencing It is also important to ensure the correct sequencing of chillers (and boilers). There is a direct correlation between outside-air temperature and the amount of chilling (or heating required). In most cases, weather compensation is either non-existent or, at best, inexact. If you need water at, say 5°C in the middle of Summer (return 10°C) you will need water at the same 5°C on a colder day when the return temperature might be 7°C. The solution is to make the sequencers target a chiller header temperature compensated to the outside-air temperature. Most chillers are also sequenced according to flow temperature. However, it makes more sense, and is therefore more logical, to sequence them to a return temperature (the same is also true with boilers). You can then ensure that there is a closer relationship between actual demand and supply or need for chilling. If you need hot water during Summer and it comes from your main heating boilers, run the boilers. However, if you have a 4-pipe system, ensure that the building heating circuits do not receiving any heat; this will only increase the load on the chillers and create a and energy-wasting conflict between heating and chilling conflict. Challenge If you have any doubts about any of these suggestions, challenge your contractors to prove that the plant that they are responsible for is operating as efficiently as it can. Getting it absolutely right, however logical, might be difficult, but unless the status quo is challenged the chances are that you will continue to waste energy on a large scale when it is easily within your grasp to do something about it. I am sure that this has a logical ring to it. Julian Miller is a director of AEC Ltd, Timbers, Picketts Lane, Nutley, East Sussex TN22 3EG.
julian@efficientcontrol.com
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